The present invention relates in general to active bolsters for occupant crash protection in automotive vehicles, and, more specifically, to an active bolster with an inflatable bladder formed by plastic wall panels joined along a hot weld seam.
An active bolster is a vehicle occupant protection device with a gas-inflatable bladder to absorb impacts and reduce trauma to occupants during a crash. As opposed to deployable air bag cushions made of various fabrics that emerge from behind various openings upon inflation, active bolsters use the interior trim surface itself to expand at the beginning of a crash event for absorbing the impact and dissipating energy through the action of an inflation gas. U.S. Pat. No. 8,205,909, issued Jun. 26, 2012, incorporated herein by reference, discloses an active knee bolster integrated into a glove box door that is light weight and visually attractive. U.S. Pat. No. 8,474,868, issued Jul. 2, 2013, also incorporated herein by reference, discloses a typical structure wherein an active bolster includes a front wall or trim panel that faces a vehicle occupant attached to a back wall or bladder member along a sealed periphery. The bladder member is further attached to a fixed reaction surface such as an instrument panel, door liner, or frame element. One or both of the walls is deformable in order to provide an expandable, inflatable bladder. For example, the bladder member may have a pleated (i.e., accordion-like) region that straightens out during inflation. The walls are initially spaced apart by a small amount when in their pre-deployment, non-inflated condition. This allows ingress of the inflation gas in a manner that achieves an even inflation across the trim panel.
The front and back walls of a typical bladder for an active bolster are comprised of molded thermoplastics such as polyethylene, polyolefin, or PVC. They are typically injection molded but can also be blow molded. When formed separately, the front and back walls must be hermetically joined around their periphery in order to form the inflatable bladder. The joint must be strong to resist separation that could result from high inflation pressures during inflation and that result when a passenger impacts the bolster. The peripheral seal is formed by hot welding, for example.
It is known that in order to optimize the dissipation of energy when an occupant contacts an active bolster, inflation gas should be vented to allow a controlled collapse of the bladder that safely decelerates the impacting occupant. Venting locations have been disclosed on the central, flat areas of the bladder wall facing the front wall, in the pleated baffle region of the bladder wall, and in welding towers that attach the bladder wall to a reaction surface, for example. It has also been suggested that an active vent can be placed in close proximity to local regions where the stresses of inflation forces may create a highest probability of weld failure.
A particular bolster design adopts various design features such as material types, weld interface, baffle size and placement, and vent configurations to obtain a desired restraint performance and weld reliability. Such a design must necessarily be based on corresponding assumptions such as a range of expected impact loads and a range of pressures of the inflation gas to be delivered by an inflator. In the event that one of these assumptions is violated (e.g., an inflator delivers a greater volume and/or pressure outside its specified range), excessive stress on the weld seam may still cause a weld separation.